Automatic frequency control (AFC) has become an important, if not essential, feature of television receivers. Conventional AFC circuits are intended to detect and correct errors in the converted frequency of the desired channel picture carrier. AFC circuits typically contain a frequency discriminating circuit that provides an output voltage related to the difference between the input signal frequency, converted to the intermediate frequency (IF), and the nominal IF picture carrier frequency, 45.75 MHz. This voltage is then applied to a voltage-sensitive reactive element in the tuner's local oscillator circuit so as to reduce the detected frequency error. Tuners utilizing varactor diodes as the frequency determining reactive elements are especially well-adapted for operation with AFC circuitry. The error correction output voltage of the AFC circuit can be readily combined with varactor tuning voltage to compensate for existing frequency errors.
Obviously one goal of an AFC system is to reduce frequency errors of the magnitude likely to occur in the operation of a television receiver and, furthermore, to provide correction so that the local oscillator frequency is sufficiently close to the correct frequency that any remaining errors in the converted IF picture carrier frequency are substantially inconsequential. In general, the AFC system is capable of providing error correction commensurate with the magnitude of error correction voltage it develops. The greater the range of error correction voltage, the greater the pull-in range. By developing an adequate amount of error correction voltage, a properly operating AFC system should be able to reduce frequency errors of .+-.1.5 MHz to less than .+-.100 KHz.
There is, however, a limit to the range of error correction voltage that may be safely developed. If an excessive error correction voltage is developed, the AFC system may be capable of locking on to signals other than the desired channel picture carrier. For example, if there is suffficient error correction voltage, the desired channel sound carrier, nominally at 41.25 MHz, and the lower adjacent channel sound carrier, nominally at 47.25 MHz, may be detected by the AFC discriminator and converted to a frequency near 45.75 MHz so as to result in a false lock-in condition. Limiting the maximum range of correction voltage reduces the possibility of such effects. The ideal range of correction voltage represents a compromise between the desire to provide as much correction as possible and the need to avoid false locking phenomena.
Striking this compromise in AFC systems with varactor tuners is particularly difficult. Because of nonlinearities in the voltage-frequency characteristic of varactor tuners, an error correction range appropriate for one channel may provide inadequate correction, or, alternatively, false locking on another. AFC circuits that develop a range of error correction voltage in proportion to the nominal tuning voltage provide a degree of compensation for these effects. It has been found, however, that alternative or improved methods of compensation are necessary and possible.